Imaging devices using X-rays, ultrasound waves and MRI (nuclear magnetic resonance method) are generally used in the medical field. The research of photoimaging devices has also been advanced in the medical field. Photoimaging devices use photoimaging technique for obtaining information on the interior of a subject by detecting light propagating inside the subject. Photoacoustic imaging technique is one of such photoimaging techniques.
Photoacoustic imaging is a technique for visualizing information relating to optical property values of the subject interior by detecting an acoustic wave (can be also referred to as photoacoustic wave), which is generated inside the subject irradiated with light, in a plurality of locations and analyzing the obtained signals. As a result, it is possible to obtain an optical property value distribution, in particular an optical energy absorption density distribution, inside the subject. A transducer using a piezoelectric effect and a transducer using capacity variations have been used as acoustic wave detectors in photoacoustic imaging. A detector using optical resonance has recently been developed (Non-Patent Literature 1).
A structure in which light is resonated between two parallel reflective plates is called a Fabry-Perot interferometer. An acoustic wave detector using the Fabry-Perot interferometer is called a Fabry-Perot probe. The Fabry-Perot probe has a structure in which a polymer film is sandwiched between two mirrors, and where an elastic wave is incident thereupon, the inter-mirror distance (referred to hereinbelow as “cavity length”) changes. The elastic wave can be detected by detecting variations in reflectance occurring in this case. In order to detect the reflectance, the Fabry-Perot probe is irradiated with a measurement light. The area where this irradiation is implemented becomes a reception region (corresponds to one element size in the piezoelectric probe) in which the acoustic wave can be detected. Since the Fabry-Perot probe has a broad band and can inhibit the decrease in sensitivity when the reception region is reduced, high-resolution imaging can be performed.
However, in practical use of imaging devices, it is important to perform imaging in a short time. In particular, when the examination object is a living body, as in the medical field, it is necessary to perform the imaging in a short time and reduce strain on the subject. In order to perform the imaging in a short time, it is necessary to perform data acquisition in a short time. However, in the use of a single-element probe, when the measurement object region is wide, the subject should be scanned two dimensionally. This lengthens the data acquisition time.
To address this problem there is an example in which, in a Fabry-Perot interferometer a CCD camera is used as a two-dimensional array sensor in order to acquire the two-dimensional distribution of an elastic wave and shorten the measurement time (Non-Patent Literature 2) in a batch mode.
A semiconductor laser called a vertical-cavity surface-emitting laser (VCSEL) is explained below. In the usual end-emission semiconductor laser, light is emitted from the end surface of the substrate thereof, whereas in the VCSEL, light is emitted in the direction perpendicular to the substrate (Patent Literature 1). Further, in the VCSEL, the reduction in volume of the active layer is achieved due to the improvement in its structure and processing process, hence the laser like this features operation a low threshold and low power consumption. Other advantages thereof include a small time constant of internal temperature increase and a fast response. In some cases, a plurality of such semiconductor lasers are arranged two dimensionally above a substrate.